Add an init service for setting sdk props

Add a oneshot init service, to be triggered early in the boot.
This is unlikely to be the permanent solution, but allows us
to get off the ground by at least getting the prop set.

Bug: 137191822
Test: m com.android.sdkext
Test: boot && adb shell getprop | grep sdk_info
Change-Id: I253c56961289d1226f3a9a471e7b6ca047c52a4e